[OpenWrt Wiki] Filesystems
https://openwrt.org/docs/techref/filesystemsImplementation in OpenWrt. The flash.layout article documents how OpenWrt uses both SquashFS and JFFS2 filesystems combined into one filesystem by overlayfs. The kernel is also stored separately from these partitions in raw flash. When the kernel is built, it is also compressed with LZMA and gzip, as documented in imagebuilder .
[OpenWrt Wiki] ZyXEL NBG6817
https://openwrt.org/toh/zyxel/nbg681721.12.2017 · ZyXEL NBG6817 Also known as ZyXEL Armor Z2 AC2600. The hardware is very similar to the ZyXEL NBG6816 (Armor Z1), but uses an all QCA IPQ8065 design (SOC and QCA9984 wlan) instead, like the Netgear Nighthawk x4s (r7800) and the Synology rt2600ac. [Zyxel NBG6817 (Armor Z2)] Supported Versions Hardware Highlights